Method for making phenol-formaldehyde resins flame-resistant

ABSTRACT

In the method described, one or more halogen-free phosphorus compounds are incorporated in the precondensed resins before the final resin-curing stage. During final curing, the phosphorus compounds are bound by covalent bonds into the resin structure and/or are trapped in the resin structure as such or in the form of aggregates or condensates.

The present invention relates to a method for renderingphenol-formaldehyde resin flame-resistant and to products containingsuch a resin. Particularly, the present invention relates to the use ofhalogen-free phosphorus compounds for making phenol-formaldehyde resinsintegrally flame-resistant.

Phenolic resins are employed as raw materials or as auxiliary agents inthe form of binders and impregnating agents for the manufacture of agreat variety of products.

The preferred fields of application of such resins are in the bonding ofderived timber products and the production of molding and coatingcompositions. Phenolic resins are furthermore employed as binders forthe manufacture of inorganic or organic fiber reinforced materialsutilized as abrasives or abrasive coatings or as materials for heatinsulation and soundproofing. Moreover, applications in the manufactureof resin papers or woven fabrics and in the manufacture of foams ofphenolic resins and other construction materials resistant to chemicalsare to be mentioned.

The phenol-formaldehyde condensation resins are acid or base catalyzedresins and are employed in the form of precondensed products called"novolak" or "resol" resins. The preparation of said products isdescribed in the chemical standard literature, see, e.g., Ullmann,Enzyklop adie der chemischen Industrie, 2nd edition, vol. 13, pages 453to 478.

Many products made by using phenol-formaldehyde resins have to meetflammability standard values. For instance, molded parts for themanufacture of cars which are made of non-woven fabrics or masoniteboards bonded with phenolic resins must satisfy the inflammabilityrequirements according to DIN 54333 and DIN 75200 as well asUS-FMVSS-302.

The requirements to be met by these molded parts and the other materialsbonded with phenolic resins mentioned above have, however, steadilyincreased over the years. For example, hardboards, particle boards andvermiculite boards for special applications such as for structures forfairs should be made flame-resistant according to construction materialclass B 1. While the requirements for being categorized in constructionmaterial class B 1 may be met by means of additives which areconventional in the plastics industry, said additives show certaindisadvantages.

Presently materials showing various chemical characteristics areemployed for providing flame-retardance. The most important examplesthereof are aluminum hydroxide, aluminum sulfate, ammonium phosphate andpolyphosphate, melamine borate, antimony oxide, red phosphorus andseveral halogen compounds, said compounds being added to the products ofphenolic resins as additives in an additional operation.

If aluminum hydroxide is to meet the requirements with respect toflame-retardance, it must be added to the phenolformaldehyde resin in aconcentration of up to 50% by weight, resulting, however, in asubstantial alteration of the physical properties of, e.g., hardboards.

Flame-proofing agents such as ammonium phosphate and sulfate arewater-soluble and may thus be leached out of said materials and finalproducts, respectively. Ammonium sulfate dissolved in molded parts basedon woven fabrics may additionally cause corrosion.

The use of ammonium polyphosphate causes problems regarding its uniformdistribution in the phenolic resin which in turn results inunsatisfactory materials bonded with phenolic resins. The requiredconstruction material class B 1 cannot be achieved with borates.Melamine borate may, however, be employed as partial component forpreventing afterglowing.

The employment of halogen compounds and of antimony oxide is undesirablein environmental respect. Red phosphorus as flame-proofing agent leadsto problems in the handling thereof.

From J. W. Lyons, "The Chemistry and Uses of Fire Retardants", 1970,pages 417-418, it is known to react phosphorus ester chlorides withphenolic bodies in order to make phenol-formaldehyde resinsflame-resistant. Said reaction proceeds under elimination of HCl gas andis thus only applicable to novolak resins. Resins of the resol-typecannot be rendered flame-resistant thereby since the liberated HCl gasadversely affects or destroys, respectively, the basic catalyst systemnecessary for the production of said resins. Furthermore, the reactionwith phosphorus ester chlorides leads to severe corrosion of theequipment. Moreover, the use of novolak resins thus renderedflame-resistant is limited by their high content of halides(particularly in cases where additionally halogenated phosphoruscompounds are employed). Particularly, in the case of a fire largeamounts of hydrogen halide gas are liberated.

It has now surprisingly been found that materials based onphenol-formaldehyde resins may directly be rendered flame-resistant ifhalogen-free phosphorus compounds (which preferably are acidic) areadded to the precondensed phenol-formaldehyde resin at any time prior tothe processing (curing) thereof for forming the final product. Therebythe hydrogen-free phosphorus compounds are distributed homogeneously inthe precondensed resin and during the further processing andapplication, respectively, of the latter said compounds become anintegral, insoluble component of the resin compositions.

According to the present invention particular advantages are affordedwhen base-catalyzed phenol-formaldehyde resins are employed since withthe present flame-proofing system hydrogen halide gases will not bereleased. Furthermore the flame-proofing system according to the presentinvention does not cause corrosion during the processing of said resinsor in the case of fire.

The present invention provides a method for making phenolformaldehyderesin and products containing said resin flame-resistant, which methodis characterized in that one or more halogen-free phosphorus compoundsare incorporated in the precondensed resin prior to the (final) curingthereof, said halogen-free phosphorus compounds being bonded to theresin structure through covalent linkages and/or being occluded(trapped) within said resin structure, as such or in the form ofaggregates or condensates, during the further processing (curing) of theresin.

Particularly suitable as halogen-free phosphorus compounds are (acidic)compounds derived from oxygen-containing compounds of the preferablypentavalent phosphorus, especially phosphoric and phosphonic acidcompounds.

Specific examples of phosphoric and phosphonic acids are ortho-, pyro-,tri- and polyphosphonic acids (preferably polyphosphoric acids) as wellas phosphonic acids having preferably 1 to 20 and particularly 1 to 10carbon atoms such as, e.g., methane phosphonic acid, hydroxyethanediphosphonic acid, 3-aminopropane phosphonic acid, 5-diethylenetri-aminopentane phosphonic acid, 4-ethylene diaminobutane phosphonicacid, morpholinomethane diphosphonic acid and2-phosphonobutane-1,2,4-tricarboxylic acid.

Preferred compounds to be reacted with the above oxygen-containingcompounds of phosphorus, particularly with said phosphoric andphosphonic acids, are hydroxyl group-containing carbon compounds whichare capable of reacting with said phosphorus compounds to formP--O--C--bonds, such as, e.g., alcohols, phenols and carboxylic acids.

The simplest and most preferred compounds of said class of compounds arealcohols and phenols which preferably are polyhydric. The alcohols maybe both aliphatic and cycloaliphatic, saturated and unsaturated, andgenerally comprise 2 to 20, particularly 2 to 10 carbon atoms, and 2 to6, particularly 2 to 4 hydroxyl groups. The aromatic alcohols (phenols)generally have a total number of carbon atoms of 6 to 20, particularly 6to 10 and preferably comprise 2 to 4 (particularly 2 or 3) hydroxylgroups directly bonded to the aromatic system. Preferably said alcoholsand phenols each form one or two P--O--C bridges with the phosphoruscompound.

Specific and preferred examples of the above alcohols are ethyleneglycol, glycerol, 1,2- and 1,3-propanediol, trimethylolpropane,1,4-butanediol, 1,2,6-hexanetriol, pentaerythritol, cyclohexanediol,meso-inositol, 1,4-dihydroxymethyl-2,5-dihydroxybenzene, pyrocatecholand resorcinol.

The polyhydric alcohols may be employed either alone or as mixture oftwo or more compounds and/or in admixture with monohydric alcohols.Preferably the corresponding final product (e.g. the ester of phosphoricacid or phosphonic acid, respectively) comprises at least one freehydroxyl group bonded to phosphorus (and preferably also at least onefree hydroxyl group bonded to carbon).

Reaction products which belong to the class of compounds described aboveand are particularly preferred according to the present invention arecompounds derived from aromatic polyols (particularly diphenols) withpolyphosphoric acid or the phosphorus-derived acids mentioned abovesince during the processing of the precondensed resins theflame-proofing system is directly incorporated in the structure of thecured phenol-formaldehyde resins.

Another preferred class of compounds which according to the presentinvention can be employed as halogen-free phosphorus compounds areheteropoly acids of phosphorus, particularly those which in addition tophosphorus comprise silicon, molybdenum and/or tungsten, as well assalts of said heteropoly acids, preferably ammonium salts. Specificexamples thereof are silicophosphoric acid, molybdenophosphoric acid andtungstophosphoric acid.

According to the present invention it has furthermore been found thatall of the compounds mentioned above exert a particularly favorableinfluence on the flame-resistance of the final products if theyadditionally contain nitrogen. In this context compounds derived fromnitrogen-containing phosphonic acids (e.g. those mentioned above) are tobe particularly mentioned.

A further class of halogen-free phosphorus compounds which may beemployed according to the present invention are (metal) salts ofphosphorus-containing acids. In this context salts of oxygen-containingacids of phosphorus (particularly of phosphoric and phosphonic acids)with trivalent cations (particularly selected from cations of aluminum,iron and chromium) and the stannous and stannic salts of said acids arepreferred according to the present invention. Specific examples of saidsalts particularly are the phosphates of the elements just mentioned.

According to the present invention it is possible to employ bothindividual halogen-free phosphorus compounds and mixtures of saidcompounds. Preferably corresponding mixtures are predominantly (e.g. atleast 75% by weight) composed of organic phosphorus compounds (e.g.esters).

According to the present invention the flame-resistance of the finalproducts is determined in the first place by their phosphorus content,based on the amount of phenolic resin. Usually the phosphorus compoundsemployed according to the present invention are used in amounts whichresult in a phosphorus content of the resin made flame-resistant of from0.5 to 15% by weight. Particularly preferred is a phosphorus content offrom 1 to 12% and particularly from 2 to 8% by weight.

Naturally, the phosphorus compounds employable according to the presentinvention show varying phosphorus concentrations. With salts ofphosphoric acid the phosphorus content is generally about 20 to 29% byweight whereas with organic phosphorus compounds a phosphorus contentranging from about 12 to 29% by weight will generally be encountered.

The flammability of the materials processed with the phenolic resinhaving been rendered flame-resistant according to the present inventionsuch as, e.g., wood, paper, non-woven fabrics, mineral fibers etc. alsoinfluences the flame-resistance of the final product.

The molecular structure of the phosphorus compounds employed accordingto the present invention also exerts an influence on theflame-resistance of the final products. Thus it has been found, e.g.,that phosphorus compounds containing P-0H groups, due to said acidicgroup contained in the molecule, dehydrate oxygen-containing organiccompounds more rapidly and more efficiently. This is particularly truefor esters of phosphoric acid which are formed from polyhydric alcoholsand tend to develop ring structures. Furthermore, as already mentionedabove, e.g., phosphorus compounds which have been made fromnitrogen-containing phosphonic acids show a higher degree offlame-proofing capacity than the corresponding phosphorus compounds freeof nitrogen at the same phosphorus content.

The inorganic phosphorus compounds (particularly salts of phosphoricacid) result in the formation of non-flammable skeleton structures whichexhibit a flame-proofing effect.

The phosphorus compounds employed according to the present invention areuniformly distributed in the precondensed phenol-formaldehyde resins ascomponent of the mixture and subsequently are firmly incorporated in theresin structure during the processing of said resins, whereby a flameprotection which is integrated in the resin is achieved and is effectiveas flame-proofing coating on the materials of phenolic resin.

As compared to the state of the art a series of advantages is achievedthereby:

The incorporation of the flame-proofing component is carried outconcurrently with the processing of the phenolic resin. Thus anadditional operation for the application of the flame-proofing system isnot necessary. The properties of the manufactured materials are notsubstantially altered thereby, with the exception of the burningbehaviour.

The flame-proofing component is fixed (chemically and/or physically) andcannot be leached out. This makes the fire protection permanent.Furthermore possible corrosion-related influences on other materials aswell as subsequent environmentally important events are eliminated,particularly due to the absence of halogen.

In contrast to the known neutral tertiary aryl esters such as, e.g.,triphenyl phosphate and tricresyl phosphate, which are highly toxic theacidic phosphorus compounds preferably employed according to the presentinvention are characterized by a low volatility in combination with aminimum toxicity.

Additionally, in specific cases the use of further condensationcatalysts may be dispensed with when using the phosphorus compoundsemployed according to the present invention.

The use of the halogen-free phosphorus compounds according to thepresent invention for making phenol-formaldehyde resins or the productsmade by using said phenol-formaldehyde resins flame-resistant is simpleand may be carried out, e.g., by admixing the halogen-free phosphoruscompounds with the precondensed resins. The mixing technique to beemployed for said purpose is determined by the type of resin and theother materials which are to be combined with said resin and arenecessary for the manufacture of the specific final products such as,e.g., particle boards, insulating boards and molding compositions.

In the processing of phenol-formaldehyde resins for making moldingcompositions the halogen-free phosphorus compounds are preferablyincorporated in said molding compositions homogenously during theproduction of the slugs, along with the flammable fillers such as, e.g.,wood flour, wood pulp, textiles etc., in a kneading device or any othercompulsory mixer. A similar technique is preferably employed whenpowderous resins are to be made flame-resistant.

When making low molecular weight resols flame-resistant the halogen-freephosphorus compounds are suitably stirred into the viscous resin matrixso as to form a single phase mixture of phenol-formaldehyde resin andhalogen-free phosphorus compound. The preparation of the mixture ispreferably carried out shortly or immediately before the resols areused.

The selection of a halogen-free phosphorus compound for a specificpurpose is determined by the intended use of the phenol-formaldehyderesin. In the case of particle boards as intended final products it ispreferred to employ products based on polyphosphoric acid,pentaerythritol and other aliphatic polyols in combination withinorganic phosphorus compounds (e.g. the salts mentioned above). In thecase of non-woven fabrics it may be appropriate to employ only theformer products.

In the manufacture of insulating boards the addition of inorganicphosphates, particularly tin(IV)-phosphate, has proven to beparticularly advantageous. The non-woven fabrics may be coated byspraying thereon the viscous precondensed resins which have beenrendered flame-resistant. Thus it is desirable for the inorganicphosphates to be present in a particle size which does not adverselyaffect the application by spraying due to plugging of the nozzle.Therefore the particle size of the halogen-free phosphorus compoundswill generally be less than 100 μm.

The following examples serve to further illustrate the present inventionwithout limiting the scope thereof in any way.

EXAMPLE 1

A phenol-formaldehyde resin of the resol-type was made flame-resistantas follows:

A halogen-free phosphorus compound was prepared by a condensationreaction of phosphoric acid, pentaerythritol and glycerol in a molarratio of 2:1:1. 100 parts by weight of the precondensedphenol-formaldehyde resin were homogeneously mixed with 30 parts byweight of the phosphorus compound and the mixture was poured into a moldthe dimensions whereof were in compliance with standard UL 94(Underwriters Laboratories).

The test samples prepared by subsequent thermal curing of the resin wererated V-O in the flammability test.

EXAMPLE 2

The mixture of phenol-formaldehyde resin and halogen-free phosphoruscompound prepared in example 1 was uniformly applied, in an amount of 55parts by weight, in 100 parts by weight of wood fibers. By pressingthereof sheet-like structures were obtained. The sheets thus preparedwere subjected to a flammability-test according to DIN 4102, part 1, andmet the requirements of the construction material class B 1.

EXAMPLE 3

For the manufacture of flame-resistant insulating materials used in themanufacturing of automobiles 100 parts by weight of preprocessed cottonwere combined with 45 parts by weight of a flame-resistant mixture ofphenol-formaldehyde resin and halogen-free phosphorus compound to form asheet-like structure. The flame-resistant resin mixture contained 62% byweight of phenol-formaldehyde resin, corresponding to a content ofphosphorus compounds of 38% by weight. The phosphorus compoundsconsisted of a mixture of 75% of aminomethane phosphonic acid ethyleneglycolester and 25% of primary tin(IV)phosphate.

According to an internal test procedure of the car manufacturer therequired inflammability of the insulating boards was achieved.

We claim:
 1. A method of making phenol-formaldehyde resins and productscontaining said resins flame-resistant, which comprises incorporatingone or more halogen-free phosphorus compounds being bonded to the resinstructure through covalent bonds or being occluded in the resinstructure or both, as such or in the form of aggregates or condensates,upon further processing of the resins, said halogen-free phosphoruscompounds being selected from the group consisting of:a) esters ofoxygen-containing acids of phosphorus with hydroxyl-group containingcarbon compounds, which esters have at least one free hydroxyl groupbonded to phosphorus; b) heteropoly acids of phosphorus and saltsthereof; and c) salts of heteropoly acids of phosphorus with trivalentcations and tin salts of said acids.
 2. The method according to claim 1,wherein said oxygen-containing acids of phosphorus are selected from thegroup consisting of phosphoric acid and phosphonic acid.
 3. The methodaccording to claim 2, wherein said phosphoric acids are polyphosphoricacids.
 4. The method according to claim 2, wherein said phosphonic acidsare C₁ -C₂₀ phosphonic acids.
 5. The method according to claim 4,wherein said phosphonic acid, is selected from the group consisting ofmethane phosphonic acid, hydroxyethane diphosphonic acid, 3-aminopropanephosphonic acid, 5-diethylene triaminopentane phosphonic acid 4-ethylenediaminobutane phosphonic acid, morpholonomethane diphosphonic acid and2-phosphorobutane-1,2,4-tricarboxylic acid.
 6. The method according toclaim 1, wherein said hydroxyl group-containing carbon compounds areselected from the group consisting of polyhydric alcohols and polyhydricphenols.
 7. The method according to claim 6, wherein said polyhydricalcohols are selected from the group consisting of ethylene glycol,glycerol, 1,2- and 1,3-propanediol, dimethylolpropane, 1,4-butanediol,1,2,6-hexanetriol, pentaerythritol, cyclohexanediol and meso-inositol.8. The method according to claim 6, wherein said polyhydric phenols areselected from the group consisting of1,4-dihydroxymethyl-2,5-dihydroxybenzene, pyrocatechol, resorcinol andmixtures thereof.
 9. The method according to claim 1, wherein theheteropoly acids of phosphorus are selected from the group consisting ofthose of silicon, molybdenum and tungsten.
 10. The method according toclaim 1, wherein said trivalent cations are selected from the groupconsisting of Al, Fe and Cr.
 11. The method according to claim 1,wherein said halogen-free phosphorus compounds are added to the resinsin an amount which results in a phosphorus content of 1 to 12% byweight.
 12. The method according to claim 11, wherein said phosphoruscontent is from 2 to 8% by weight.
 13. The method according to claim 1,which further comprises curing said resin.
 14. The method according toclaim 13, wherein prior to curing the resin, a filler is added.
 15. Apre-condensed phenol-formaldehyde resin, obtained by the method ofclaim
 1. 16. A method of producing molding compositions, coatingcompositions, binders, impregnating agents and adhesives, whichcomprises incorporating the pre-condensed phenolformaldehyde resinsaccording to claim 15, thereunto during production thereof.